Engine: Per-map art previous board filename is not updated when starting a map without loading a save

Forum Thread: https://forums.duke4.net/topic/10927-per-map-art-oversight-on-savegame-load/

When a new map is started, the per-map art tiles are cleared or overridden, but the previousboardfilename variable is not updated.

This can cause a problem when one does the following:

1. Load a save of a map with per-map art defined. previousboardfilename is updated to the current map's filename.
2. Start a new map through the newgame menu, or transition to another map through level exit. The per-map art tiles are cleared.
3. Now load the save. Since previousboardfilename still holds the filename of the map in the save, the per-map tiles are not reloaded. This can result in a hall-of-mirrors effect, or the wrong textures appearing in place of the per-map tiles.

This variable was originally devised as a band-aid fix to cache pollution when savegames of maps with per-map tiles defined were loaded repeatedly. A solution would be to update previousboardfilename on every map load, and not just when a savegame is loaded.

Here's the map to reproduce the problem: https://forums.duke4.net/index.php?app=core&module=attach&section=attach&attach_id=14516